Camshaft phasers (also referred to herein as “cam phasers”) are used to control the angular phase relationship of a camshaft to a pulley or sprocket driven by a crankshaft of an internal combustion engine. A variable cam phaser (VCP) allows changing the phase relationship while the engine is running. Typically, a VCP is used to phase shift an intake cam on a dual cam engine to broaden the torque curve of the engine, to increase peak power at high revolution speeds, and to improve the idle quality. Also, an exhaust cam can be phase shifted by a cam phaser to provide internal charge dilution control, which can significantly reduce HC and NOx emissions, and/or to improve fuel economy. The above objectives are known in the art as “combustion demands”. With this definition, a VCP is used to account for combustion demands of an internal combustion engine.
Cam phasers typically are either “vane-type” or “spline-type” and are controlled by hydraulic systems that use pressurized lubrication oil from the engine. An “advance” or “retard” position of the camshaft is commanded via an oil flow control valve (OCV) that controls the oil flow to different ports entering a VCP. A typical OCV is a spool valve comprising a spool slidably disposed in a cylindrical body, the spool being driven by attachment to an electrical solenoid, and the position of the spool within the body serving to open and close appropriate porting in the body connected to the VCP ports.
A known problem in cam phaser operation is that valve-opening and valve-closing events of a camshaft can give rise to torque reversals resulting in pressure peaks in the oil contained in the chambers of the VCP, which peaks can be higher than the oil control supply pressure, i.e., the oil pressure supplied by the engine. The camshaft experiences a measurable resistance to rotation as a cam follower climbs the opening ramp of each cam lobe; and similarly, a measurable assistance to rotation as the cam follower descends the closing ramp. Thus, the measurable torque on the camshaft follows a predictable variation, the amplitude and frequency (period) of which are governed by the particular size and shape of an engine's cam lobes, drive train kinematics and the number of cylinders.
This phenomenon can lead to a certain amount of undesired reverse oil flow out of the phaser, diminishing the phasing performance of the cam phasing system by causing the rotational position of the phaser rotor within the phaser stator to be changed. This problem is exacerbated by any air bubbles in the oil supply system, which act as pneumatic cushions preventing hydraulic rigidity, and further by conditions of high oil temperature and/or low oil pressure such as may be experienced by an engine under heavy load and thermal stress.
To avoid such reverse oil flow under the above mentioned circumstances, it is known in the art to employ a check valve (CV) in the oil supply passage of either the cylinder head or the crankcase. Such a check valve also ensures that the cam phaser does not empty out in cases when the oil pressure is reduced, for example when the engine is stopped. However, this approach adds significant cost to the cylinder head or engine block. Also, the implementation of the check valve can be difficult because of oil routing. Further, the check valve should not be placed too far away from the cam phaser in order to be effective. Perhaps most important, a mechanical check valve itself acts as a flow restriction in the allowed direction of oil flow, thus diminishing the inherent phasing rate (response rate) performance of a cam phaser system.
Another approach, as disclosed in Published US Patent Application No. 2007/0175425, published Aug. 2, 2007, the relevant disclosure of which is incorporated herein by reference, is to vary the action of the solenoid driving the OCV to effectively block such reverse flow by appropriate positioning of the OCV spool within the valve body. The OCV can be cycled full on and full off (preferably “dithered” at an intermediate spool position by application of an AC signal to the solenoid) in synchronization with cam torque reversals, which function serves to accept all positive hydraulic power into the VCP, which enhances the phasing rate, while rejecting all negative hydraulic power, which diminishes the phasing rate, thus effectively rectifying oil pressure in the VCP. The OCV thus can function as a “virtual check valve” (VCV) when a constant phase angle of the rotor is desired, allowing omission of an additional mechanical check valve, in addition to its nominal function of providing oil selectively to the advance and retard ports of the phaser when a change in phase angle is desired.
The required frequency of OCV activation is readily calculated from the engine speed and the number of engine cylinders as follows:FHz=(RPM×# of cylinders)/120  (Eq. 1)
However, correct synchronization timing between OCV spool motions and cam torque reversals has been not at all easy to accomplish in the prior art, nor is a method for accurate synchronization disclosed in detail in the incorporated reference.
What is needed in the art is a method for providing accurate synchronization between OCV spool motion and cam torque reversals in an internal combustion engine to effectively block oil flow reversals from a camshaft phaser during camshaft torque reversals.
It is a principal object of the present invention to improve operation of a camshaft phaser and of an engine incorporating a camshaft phaser.